This invention relates to a method of processing non-rigid materials and more specifically, a method for processing carbon fiber material.
Carbon fibers represent one of the most unique products having the strength of steel, the weight of aluminum and the conductivity of copper. These properties are extremely desirable for use today in building and construction, i.e., the automotive industry and in electronics and telecommunications products. They possess excellent thermal and electrical conductivity and because of their lightweight property, are ideal for the aircraft industry.
However, because of the relatively high cost of synthetic-based carbon fibers, their use has been limited. Only high priced goods such as aircraft, sporting goods and other expensive items could afford the use of carbon fibers. Other applications that require strength, stiffness, low weight and good fatigue characteristics had to use other less expensive and less effective products than carbon fiber. Because of these drawbacks or disadvantages, there is a real need for a carbon fiber that can economically compete with these cheaper and less effective materials. A process that will reduce production costs of carbon fibers and make them more universally available is very desirable. Any improved process to create stronger and lighter products of petroleum-based carbon fibers will also create new business and redefine how business is done in the carbon fiber industry. The present invention provides a method for processing carbon fibers that will meet these ends.
There are known processes for treating carbon fiber materials such as those described in U.S. Pat. Nos. 5,283,113 (Nishimura et al) and 5,967,770 (Heine et al). Other products not concerned with carbon fiber but directed to treatment of fibrous uses are 4,676,445; 4,504,344; 5,915,613; 5,979,731; 6,003,750; 6,004,432; and 6,050,469. Other patents directed to conveyor type processing of web materials are 4,718,543; 4,911,286; 5,791,455; and 5,848,890.
These above patents concerned with the unique product carbon fiber both disclose the basic process for producing carbon fiber felt which is the starting point for the process of the present invention. Nishimura discloses a process for continuously producing a pitch-based carbon fiber felt. His process starts with the raw materials and produces a pitch based carbon fiber felt having uniform unit weight and good physical properties. The Heine patent discloses a continuous treatment of carbon fibers in polyacrylonitrile which are heated and processed to produce a carbon fiber strand for use in the plastics arts.
The present process begins with a product similar to that of the Nishimura patent. Basically the present process takes a fiber mat of carbon pitch fibers and passes them on a conveyor into a section of the furnace that transfers it to the specialized sections to carbonize the fibers. This process will be further defined in reference to the accompanying drawings.
It is therefore an object of this invention to provide a process of treating carbon fiber mats devoid of the above noted disadvantages.
Another object of this invention is to provide a more efficient method of heating a fibrous carbon fiber mat.
Still a further object of this invention is to provide a process for treating carbon fiber mats that will allow for higher carbon fiber production and ultimate lower sale price.
Yet another object of this invention is to provide a very efficient and effective process for heating and transferring carbon fiber mats.
These and other objects of this invention are accomplished by a process for treating carbon-fiber mats that utilizes various treatment stations and effective transfer of the mat from one conveyor to another.
Processing of carbon fibers requires high temperatures that standard alloy conveyors may not be able to withstand. High temperature conveyors are costly and fragile. If such conveyors are made of carbon or graphite, they are subject to oxidation if not operated in an inert atmosphere. Portions of the process may require an oxidization atmosphere, which would not allow the use of carbon or graphite conveyors or components to transport the material. The high temperature conveyor could be used only in the required area, and other conveyors could be used before and after that area.
The material heating section may be a standard furnace. Since the carbon fiber material being processed is lightweight and porous, it acts as a thermal insulator and it may take significant time to heat through the entire material by radiation and conduction alone. This design shows heating by radiation with gas flows providing convection and driving the heat through the material. This would reduce the time required which would allow faster conveyor speed and/or a shorter heating section.
Heating elements heat the surface of the material by radiation. Gas flows through the hot surface and is heated, converting radiant heat to convection heat thus driving the heat deeper into the material at a faster rate.
As an option, radiation plates could be used to protect heating elements from the gas flow (which could erode heaters since heaters may run much hotter).
Another option is to add a porous plate above the material being processed to absorb the radiant heat and convert it to hot gas, which heats the material when passing through it.
Heaters may be provided below the conveyor to aid in heating the material in contact with the conveyor.
The transfer of lightweight fibrous material (such as carbon fibers) from one conveyor to another can be difficult if the material is fragile, very lightweight, composed of layers, or not able to bend sharply. This transfer section uses gas flow (air, nitrogen, etc) from a blower to xe2x80x9cfloatxe2x80x9d the material across the gap between conveyors. A drum or other moving conveyor above the gap allows the material to be blown or drawn up to the drum. The pressure under the material keeps the material from falling and delaminating as it crosses the gap.
The top drum could be a cylindrical perforated drum that rotates. Non-rotating ducts within the drum allow the suction to be in the area desired. Additional internal ducts could provide for xe2x80x9cblow offxe2x80x9d through the drum, which aids in removal of the material from the drum. This xe2x80x9cblow offxe2x80x9d also acts as a self-cleaning device to remove fibers sucked into the drum perforations.
The top drum could have the suction connection to the blower out the side of the drum, or the gas could flow through the drum surface into a mating duct.
The top drum could be replaced with a flat (or arched) conveyor belt. This would keep the material from bending if so desired. The flat conveyor could have internal ducts as described above. The top conveyor could be driven by conventional methods.
A perforated scraper could be added to scrape the material from the first conveyor if needed. This scraper would allow gas to flow through it to blow the material off the first conveyor and up to the drum. By positioning the scraper as shown, it will help guide the material to the inclined second conveyor if the gas flow should stop.
Exit purge boxes are used to allow control of gasses from one conveyor area to the other. For example, the first conveyor could have air as a gas while the second conveyor could have nitrogen as a gas. The exit purge box shown has provisions for a nitrogen purge pipe, which sprays nitrogen across the moving material. This could be on the entrance purge box also, if desired. The purge boxes have movable tops, which can be externally adjusted up or down to suite the height of the material. The boxes are designed to be remotely closed to compress the material and reduce gas flow from one conveyor to the other, or slammed down for safety reasons as needed.
Fibrous materials like carbon fiber and ceramic fiber mats have low strength and cannot tolerate abrasion. They must be placed on a conveyor to take them through furnaces for thermal processing.
In one embodiment of this invention, the large drum in the process of this invention has a non-rotating inner spool, which is hollow. The suction side of a blower is connected to this inner opening of the spool. The spool is secured from rotating by being locked in the wall. There are slots cut in the spool to allow gas to flow through holes in the drum hub and into the appropriate segments created by vanes. As the drum rotates, vane sections become xe2x80x9copenedxe2x80x9d and xe2x80x9cclosedxe2x80x9d to the suction of the inner spool.